The present invention relates to an excitation circuit for exciting ultrasonic transducer elements.
Recently, there has been developed an ultrasonic diagnostic apparatus of electronic scanning type in which use is made of a probe composed of a plurality of strip-like ultrasonic transducer elements disposed in an array. For having a better understanding of the invention, description will first be made in some detail on the principle of the ultrasonic diagnostic apparatus of linear scan type among those of the electronic scanning type. Referring to FIG. 1, k transducer elements 1 (hereinafter referred to also as element) which are simultaneously excited are grouped into one set, where k is equal to 4 in the case of the illustrated example. The excitation of the transducer elements grouped in one set is then sequentially changed over from one to another succeeding element on the one-by-one basis so that the ultrasonic beams produced by the selectively grouped elements to be excited simultaneously are shifted progressively in the direction in which the transducer elements are arrayed in a linear row. In other words, the ultrasonic beams are sequentially emitted in a linear order, as indicated by scanning lines L.sub.1, L.sub.2, . . . , L.sub.n-4+1 in FIG. 1. By the way, when the number of all the transducer elements 1 is represented by n, the total number M of the scanning by the ultrasonic beams can be expressed by n-k+1, where k represents the number of the transducer elements excited simultaneously, as defined above. Assuming that n=100 and k=4, a first scanning line L.sub.1 is obtained by driving pulsers P.sub.1 to P.sub.4 connected to the transducer elements #1 to #4. A second scanning line L.sub.2 is produced by driving the pulsers P.sub.2 to P.sub.5 connected to the elements #2 to #5. By repeating sequentially the driving of the pulsers in the similar manner, 97 scanning lines can be obtained in total along the transducer element array. Under the circumstance, when the number of the scanning lines is to be increased, a correspondingly increased number of pulsers are required, which of course means high expensiveness of the scanner or probe. Accordingly, in the practical applications, a switching circuit is employed with a view to decreasing the number of the pulsers to a possible minimum. FIG. 2 shows an arrangement including a switching circuit to this end. Referring to the figure, the transducer elements 1 and the pulsers P.sub.1 to P.sub.4 are interconnected through a matrix-like switching circuit in which the individual switches S.sub.1 to S.sub.4, S.sub.5 to S.sub.9, . . . , S.sub.n-3 to S.sub.n are provided at the cross-points between the lines outgoing from the pulsers and the lines leading to the transducer elements for every group of the elements. It will be seen that the arrangement shown in FIG. 2 allows the number of the pulser P to be decreased to that of the elements excited simultaneously at minimum. In this connection, it should however be mentioned that the pulse voltage produced by the pulser for exciting the transducer element which is generally a piezo-electric type in the case of the ultrasonic diagnostic apparatus is usually in a form of a burst waveform signal of about peak-to-peak 100 V in a frequency range of 1 to 10 MHz. Consequently, in order to switch the pulses of the positive and negative polarities having such high frequency and voltage, a high-voltage-rated switching element capable of conducting the pulse signal with both polarities is required for the switch S.sub.1 to S.sub.n. Such switching element or device may be realized in an arrangement shown in FIG. 3 in which diodes D.sub.1 and D.sub.2 are connected in an anti-parallel connection so as to pass and block the pulse signal of both polarities. With such circuit arrangement, however, a control circuit of a complicated configuration is required for applying high control voltages V.sub.a and V.sub.b of positive and negative polarities to the control terminals T.sub.1 and T.sub.2 of the diodes D.sub.1 and D.sub.2, respectively, and vice versa. Further, the signal passing through the switching element of this type will disadvantageously be subjected to distortion upon transition of the polarity of the signal, i.e. in the vicinity of the time point at which one diode is turned off (or blocked) while the other is turned on (conductive). The disadvantage of this kind will further be described below by referring to FIGS. 4a-4c.
In FIG. 4b there is shown a circuit in which the switching element is constituted by a conventional diode D which is conductive only in one direction. When the control voltage V.sub.c applied to the diode D through a terminal T is sufficiently lower than a minimum value -V.sub.x of an input pulse signal shown in FIG. 4a, the diode functions to block the input pulse voltage. In other words, the switching element is in the blocked or non-conducting state. On the other hand, when the control voltage V.sub.c is higher than the peak voltage V.sub.p of the input pulse signal, the latter can pass through the diode D. In other words, the diode switch is in the conducting state. Accordingly, in the case of the switch circuit shown in FIG. 4b, the control voltage has to be higher than the highest level of the input pulse signal and lower than the lowest level thereof. To this end, a control voltage having a large amplitude in both positive and negative regions is required, involving a complicated control circuit and thus making impractical the switching circuit shown in FIG. 4b. It should additionally be mentioned that when the control voltage V.sub.c is of a single polarity, e.g. varies between zero and the level -V.sub.x, the output voltage from the switching circuit exhibits a rectified waveform corresponding to the input pulse voltage having the positive amplitude range blocked, as shown in FIG. 4c.